From the viewpoints of power saving and prolonging the life, vehicular lighting devices equipped with light emitting diodes (LEDs) are becoming widespread.
The voltage applied to a vehicular lighting device fluctuates. Accordingly, in the vehicular lighting device, an operating voltage range (voltage fluctuation range) is defined.
However, if an electrical connection in a power supply circuit is temporarily disconnected due to vehicle vibration or repair works, an excessive voltage (surge voltage) exceeding the operating voltage range may be applied to the light emitting diode.
Consequently, a Zener diode is provided on an input side of the vehicular lighting device so that a voltage exceeding a predetermined voltage is not applied to the light emitting diode even if an excessive voltage is temporarily generated in the power supply circuit.
However, if a solution is merely that a constant voltage is made by the Zener diode, the Zener diode with high allowable power is required. The Zener diode with high allowable power increases in a size. As the size of the Zener diode increases, providing the Zener diode on a substrate on which the light emitting diode is mounted becomes difficult. In recent years, a downsized vehicular lighting device is demanded, and thus providing a large-sized Zener diode in the vehicular lighting device becomes further difficult.
Accordingly, a voltage clamping circuit having the Zener diode is generally provided on an output side of a power supply circuit that supplies electric power to the vehicular lighting device. However, in this configuration, a wiring between the Zener diode and the light emitting diode becomes long, and protection effect against the excessive voltage may be insufficient.
Therefore, development of a technology capable of improving the protection effect against the excessive voltage is desired.